Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: MikeSattler on August 02, 2017, 05:39:53 AM
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This isn't a major issue but one I'd like some guidance on solving :)
There is a slight buzz in the right channel that increases with volume (it becomes audible around 12 o'clock on the pot) and increases until the pot is at max and then it disappears completely.
I've pretty much ruled out the RCA inputs and the pot itself as the culprit by using shorting jacks and re-soldering the wires leading to the pot (the noise didn't move over to the left channel).
Where would be the next place to start looking for the culprit?
Thanks!
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I've seen some folks pop in mentioning a right channel buzz similar to what you're mentioning, and it has either turned out to be loose transformer hardware or an improperly soldered wire between the tab on the chassis and the lug of the 6 lug T-strip that it grounds.
Other possibilities can include a tube that needs 50 or so hours of run-in time, or potentially a different solder joint in the amplifier that isn't all the way cooked.
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Thanks for the suggestions! By loose transformer hardware do you mean the 4 screws that are mounting the transformer to the chassis?
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Yes.
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Thanks for the help so far guys. I just finished re-soldering every joint (including the speedball boards) and this buzz is still there :(
I've also tried multiple sets of tubes and that didn't solve anything either.
What would the next step be here?
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Did you tighten up all the screws?
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Yup, forgot to include that in the last post.
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(the noise didn't move over to the left channel).
I'm not following this. Why would you assume that the noise would change channels after reflowing the solder joints. Did you swap wires?
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Thanks for the help so far guys. I just finished re-soldering every joint (including the speedball boards) and this buzz is still there :(
I've also tried multiple sets of tubes and that didn't solve anything either.
What would the next step be here?
Have you checked your interconnects? Maybe swap left to right?
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Get a wood chopstick and plug in an inexpensive pair of headphones. Poke around in the circuit until the noise changes in order to find where the issue may be.
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I'm not following this. Why would you assume that the noise would change channels after reflowing the solder joints. Did you swap wires?
Hey Doc! Yup, the first thing I did was to flip the RCA inputs on the pot to see if the noise was coming from the RCA jacks. Then, I flipped the outputs on the left/right channels on the pot to see if it was coming from the pot itself.
I just went through and did the chopstick test and the right buzz didn't seem to change. However I did notice that while i was poking around on the 9 pin socket I'm getting ground hum when I touch the left/right input and output joints on the socket. I'm also getting the same hum when I touch the 12au7 tube with my hand, or when I touch the volume knob so something weird is going on there.
I'm using an alps blue velvet pot with a metal washer on the top/bottom of the chassis to screw it down. Is my chassis forming some sort of grounding issue with the pot?
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Yeah, it sounds like it could be the pot. Does the hum change character when you turn the pot up and down with no signal present?
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The ground hum doesn't change character, but I did just realize something: when I touch nothing but the 12au7 tube I hear the ground hum, and when I keep my hand on the 12au7 and touch the chassis or pot with my other hand it goes away.
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That also tends to indicate an issue at the grid of the 12AU7, which is of course connected to the output of the pot. If the hum didn't move when you changed the output connections from the pot to the 12AU7, you might want to reflow the joints at the 12AU7 socket and the pot. Maybe also clean the tube pins.
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This still potentially points to loose hardware on the transformer or #8 screw by the power entry module. A missed solder joint on that small wire that ties the three together would also be a possibility.
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I tightened down all the transformer hardware as well as the ground screw on the chassis last night. Used some needle nose pliers and a screwdriver so it's about as tight as it's going to ever be. I also re-soldered the pot and 9-pin socket just in case I missed a joint last time, and cleaned out all the tube pins with de-oxit but nothing has worked so far.
Just to clarify, there's 2 separate noises going on: one is a buzz in the right channel that increases with volume, and the other is a 60hz ground hum when I touch the input tube. The ground hum goes away when I touch the chassis.
I tightened down all the other socket screws and hardware just in case it was another loose screw but that didn't help either.
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The ground hum goes away when I touch the chassis.
Do you have 3 prong outlets in your home? Have you put an outlet tester on them to check that the ground is actually there?
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Thanks for the help so far PB!
Yup my outlets are all grounded. I've ruled out any sort of interference or outside noise since the amp was working perfectly for years in the same room and now all of a sudden is having issues.
I went through and traced the ground connections according to Graingers sticky and had good ohm readings on all of them so I'm not sure what else to do at this point. My last guess is that the 9-pin socket may be bad so I'm going to order a replacement and solder that in to see if it fixes it. I have been doing a lot of tube rolling lately so maybe one of the pins on the adaptor wore out.
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FIXED
Wow... So it turns out the brand new Alps Blue velvet pot I bought was bad. It must have had something wrong with the ground connection in the pot since the noise never switched channels when I re-wired the pot. I ordered a new one just on a whim and installed it, and now it's fixed!
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These are easily damaged if they aren't paired with a small PC board that has external wire pads for the connections in the amplifier.
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Yea that makes sense, the prongs don't seem very sturdy at all. Since this is now the third Blue velvet I've had to install in here I might just make the leap to a good stepped attenuator instead.
Anyways, thanks for all the help troubleshooting!
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Some Blue Velvets have eyelets instead of pins. Even if you don't use the hole it gives you more surface area for a lay on joint.